Oligodendrocytes are myelin-forming cells in the central nervous system (CNS). Oligodendrocyte development, consisting of oligodendrocyte differentiation and myelination (axon wrapping by myelin sheath), is under precise regulation. Furthermore oligodendrocyte regeneration and remyelination are frequently dysregulated, leading to demyelinating lesions in demyelinating diseases such as multiple sclerosis. Therefore, it is important to investigate how oligodendrocyte differentiation and myelination are regulated during normal development and dysregulated in demyelination diseases. Our long term goal is to study the underlying mechanisms that regulate OL differentiation and remyelination. The expression of the Wnt effector transcription factor 7-like 2 (TCF7l2, also known as TCF4 in cancer research) in multiple sclerosis lesions is one such mechanism. Previous studies from others and our own laboratory have shown that the canonical Wnt/?-catenin signaling pathway inhibits oligodendrocyte differentiation (review Guo et al., 2015). Therefore, it has been naturally assumed that TCF7l2 inhibits OL differentiation and myelination through transcriptionally activating Wnt/?-catenin signaling. However this assumption is difficult to reconcile with the observation of oligodendrocyte development in TCF7l2-null mutants which die after born. We recently reported that conditionally ablating TCF7l2 by Cre-loxP genetic approach inhibits oligodendrocyte differentiation and does not perturb Wnt/?-catenin signaling activity (Hammond et al., 2015). Based on these genetic data, we propose an alternative hypothesis that TCF7l2 promotes postnatal oligodendrocyte differentiation and development by mechanisms independent of Wnt/?-catenin signaling. Our recent and preliminary studies suggest a stage-dependent role of TCF7l2 in oligodendrocyte development: it initiates oligodendrocyte differentiation and may also promote downstream myelination. In Aim 1, we will conditionally ablate TCF7l2 in already differentiated OLs to test our hypothesis that, in addition to regulating oligodendrocyte differentiation, TCF7l2 is required for developmental myelination and remyelination after myelin damage. We further propose that TCF7l2's function as a gene repressor plays a crucial role in regulating oligodendrocyte differentiation. In Aim 2, we will use in vivo gene ablation and in vitro culture system to test the novel hypothesis that TCF7l2 positively regulates the timing of oligodendrocyte differentiation by repressing the differentiatio inhibitory pathways. Our studies will likely provide novel data on the dual role of TCF7l2 during OL differentiation and (re)myelination and will change our conventional understanding of TCF7l2 in oligodendrocyte development. Our project will also provide new insights justifying that future therapeutic designs should attempt to enhance, rather than inhibit, TCF7l2 to attenuate or overcome arrested oligodendrocyte differentiation and myelination in dysmyelinating and demyelinating diseases.